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Cambrian Rocks of East Point, Nahant Massachusetts

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Cambrian Rocks of East Point, Nahant Massachusetts University of New Hampshire University of New Hampshire Scholars' Repository New England Intercollegiate Geological NEIGC Trips Excursion Conference Collection 1-1-1984 Cambrian rocks of East Point, Nahant Massachusetts Bailey, Richard H. Follow this and additional works at: https://scholars.unh.edu/neigc_trips Recommended Citation Bailey, Richard H., "Cambrian rocks of East Point, Nahant Massachusetts" (1984). NEIGC Trips. 354. https://scholars.unh.edu/neigc_trips/354 This Text is brought to you for free and open access by the New England Intercollegiate Geological Excursion Conference Collection at University of New Hampshire Scholars' Repository. It has been accepted for inclusion in NEIGC Trips by an authorized administrator of University of New Hampshire Scholars' Repository. For more information, please contact [email protected]. Cl-1 249 CAMBRIAN ROCKS OF EAST POINT, NAHANT, MASSACHUSETTS Richard H. Bailey Department of Earth Sciences Northeastern University Boston, MA 02115 Introduction The quintessential stratigraphic component of Avalonian terranes of eastern North America is a Cambrian succession bearing the so-called Acado-Baltic trilobite assemblage. Spectacular sea cliffs at East Point, the easternmost extremity of Nahant, Massachusetts, afford an opportunity to examine a continuous and well exposed Lower Cambrian section on the Boston Platform. The Nahant Gabbro, sills, abundant dikes, and faults cut the Cambrian strata and add to the geological excitement. Indeed, it is difficult to move more t h a n a few meters along the cliffs without discovering a feature that will arouse your curiosity. This is also a wonderful place to watch waves crash against cliffs and to stare across the Atlantic in the direction of Africa. Regional Lower Cambrian Stratigraphy Cambrian rocks of Nahant are correlated with distinctive green and red nodular slates of the Weymouth Formation (LaForge, 1932) exposed in the Mill Cove area of Weymouth (Figure 1). Trilobites have never been found in Nahant Strata (a slate pebble with Strenuella was found; Grabau, 1900), but a thin zone in the Weymouth Formation at the type locality (Burr, 1900; Grabau, 1900) yielded a diagnostic trilobite assemblage (Strenuella, Callavia, and Weymouthia) indicative of the late Early Cambrian of the Acado- Baltic faunal province and similar horizons in England and Morocco (Shaw, 1950; Theokritoff, 1968; Anstey, 1979). The Hoppin Formation at Hoppin Hill near North Attleboro, (Figure 1) also contains Strenuella strenua (Billings). A number of workers have noted (Foerste, in Shaler and others, 1899; Grabau 1900; Theokritoff, 1968; Landing and Brett, 1982) that in both the Hoppin and Weymouth Formations the trilobite bearing assemblages overlie or are separated from a fauna dominated by small conoidal fossils. Landing and Brett (1982) assigned the lower part of the Hoppin Formation, and the limestones of the Weymouth Formation at N a h a n t which lack trilobites, to the earliest Cambrian Tommotian Stage (Figure 1) and the microfauna associated with the Strenuella ((trilobite) bearing beds at Hoppin Hill to the overlying Atdabanian Stage. The restriction of faunas to particular lithofacies, the very limited exposure of good stratigraphic sections, the intimate association of the trilobite and non-trilobite assemblages, and the occurrence of a trilobite (?) in limestone at Nahant suggest to me that the Cl-2 .£ ft -P -p£ 0) Ifl o o CO p i £ c d o g : • - - h pq — -p C u '—' a) CTN•p co r- £ & rd (d O H ,£ £ *H cd ,£ - 2 £ a >1 O (0 0) TJ •H ft -P £ -P Co co cd td•H £ O -P ril O CO PI ft -P O '­ CO er* •H £ ffi rd CD - H £ O ft ft -H N o £ ,£ Cft £ co £ a a) ■h id O f t a ft UK O M O u 00 a> CD CD a> CD O a> k » o X) a CO o o > o CD o co TD 7 3 7 3 O O a> CD CD c <x> o o a> >* k . w o a CD a> a> 5 o E g > > 7 3 O O c CD o O O cn O o w . o o E in a> CD m a> £ o a> (JL o CO a co CD 5 CD O O E CO * o o c a> co - 3 CO N CO o o o CL e E <u CD o <D o O O <D o cd a> £ ro O' o c r (sj9|aui) ss9u>|0jqi Cl-3 251 Lower Cambrian rocks of eastern Massachusetts are probably not Tommotian but lower Atdabanian or slightly younger. Theokritoff (1968) explained the lack of co-occurrence of the trilobite and non-trilobite faunas as a probable result of facies control of assemblages and he therefore suggested that a non-trilobite Coleoloides fauna need not necessarily be earliest Cambrian. Landing, Nowland, and Fletcher (1980) also report the extension of several typical Tommotian phosphatic microfossil ranges well into the Callavia zone in Nova Scotia. We clearly need a better understanding of the taxonomy, stratigraphic ranges, and paleoecology of many of the non-trilobite index fossils of the earliest Cambrian. Until such knowledge is gained I suggest that application of the stage names of the Siberian Platform (Matthews and Missarzhevsky, 1975; Raaben, 1981) to the Cambrian successions of isolated areas such as eastern Massachusetts may be somewhat premature. Strata equivalent in tectonic setting, depositional environment, and fossil content to the Hoppin and Weymouth Formations are excellently exposed in eastern Newfoundland and in more limited exposures in Nova Scotia and New Brunswick (McCartney, 1969). In Newfoundland and New Brunswick there is a profound, often angular, unconformity, with an overlying basal quartzarenite, as at Hoppin Hill, followed by the Lower Cambrian succession (Skehan, 1969) . Kaye and Zartman (1980) have recently proposed that the Cambridge Formation of the late Precambrian Boston Basin grades upward into the Weymouth Formation to form a continuous and conformable succession. The lithologies of the Weymouth and the Cambridge are quite distinct and different, especially when viewed on the outcrop scale. The well documented nonconformity at the base of the Cambrian at Hoppin Hill (Billings, 1929; Dowse, 1950), less than 40 km to the southwest of the Boston Basin, is the result of transgression of shallow water facies onto a stable continental(?) block. The shallow water facies of the Weymouth and Hoppin Formations, the uncomformable association with stable basement, and the regional correlations do not support a basinal setting for the Weymouth Formation. Billings (1982) argues, on structural grounds, that an uncomplicated transition between the Boston Bay Group and the Cambrian strata in Weymouth and Quincy is unlikely. $ Nahant Stratigraphy About 130 m of strata are exposed in northerly dipping beds of East Point. It is difficult to present a simple stratigraphic section as there are numerous fault offsets and 2 thick sills that interrupt the sequence. The section consists of a dark silicified mudstone or argillite with interbedded nodular horizons and limestone beds. A 3m thick limestone bed is found on the southeast side of the point along the top of the cliff. The three basic Cambrian lithologies are discussed below. Cl-4 252 Argillite Most of the section is a brittle, dark gray to black, thinly laminated argillite. Bedding is faintly visible in hand specimen. Quartz silt and very fine quartz grains are present as thin, parallel laminae, and lenses (0.1-0.5 mm thick) or as scattered isolated grains. These laminae are separated by a dark structureless mudstone. Some undulating contacts resemble scour surfaces. Very rare burrow-like structures are present. Nodular Argillite The most characteristic lithology of the Weymouth is a red, green, or black slate or argillite containing elongate carbonate nodules. The nodules are from 0.5 to 3 cm in thickness and most are about 3 to 15 cm in length. They occur with varying frequency along bedding planes and occasionally are so abundant as to comprise a thin "limestone" bed. The nodules have been altered primarily by silicification, and by replacement with Ca-garnet, tremolite, wollastonite, and epidote. The highly altered nodules are often zoned or banded with chert and carbonate interiors and garnet and Ca-silicate rims (Bingham, 1977). Chertification initiated in the nodule often extended into the surrounding mud­ stone. I have seen no fossils in either the replaced or the relatively unaltered nodules. Underlying and overlying laminations in the mudstone are often deflected and appear to be displaced by nodules suggesting that the mudstone was unconsolidated when the nodules formed. The shapes of nodules are reminiscent of algal structures, but no definitive internal structure is present. Limestone White to light gray limestone beds range in thickness from 3 cm to 3 m. The very thin limestones usually occur in groups with interbedded, abundantly nodular, slate or argillite. Thicker limestones contain 0.5-3 cm thick, very thinly laminated, highly irregular, brownish or greenish chert beds. The cherts themselves do not contain fossils; however, patches of fossiliferous chertified limestones are associated with the greenish layers. Fossils occur in clearly defined thin zones within the limestones. These fossiliferous layers may be continuous and traceable for several meters along the surface of the outcrops or they may be isolated as irregular masses. Petrography of Limestones The primary textures of limestones at East Point are remarkably well preserved given the proximity of igneous rocks. The rocks were originally biomicrites or fossil wackestones. The micrite has generally recrystallized to very finely crystalline sparry calcite although some patches of microspar are present (Figure 2). Abundant bioclast, primarily hyolithids, are composed Cl-5 253 > V M 5 mm 5 mm Figure 2 Negative prints of acetate peels from slabs cut parallel to bedding. Microfacies labeled micrite (M), biomicrite (BM), biosparite or packstone (BS), scale bar is 2 cm.
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